The measurement of total serum thyroxine (T4) is used to help diagnose thyroid condition and evaluate therapy. The assay is performed in vitro thereby avoiding administration of radioactivity to the patient. Only a blood sample of the patient is required. The T4 assay, employing a radioactive isotope, permits evaluation of thyroid function under certain circumstances in which other standard methods such as the measurement of protein bound iodine may not be applicable due to the presence of various forms of iodine. Radioimmunoassay T4 procedures may be used following administration of iodine containing compounds and can also be used during the course of treatment with thyroid medication (in general more specific than other tests). The measurement of total serum thyroxine offers a means for measuring the direct output of the thyroid gland as thyroxine.
T4 assays require a means for extracting T4 from plasma proteins (alcholic solvents are extensively used for this purpose) and a method for separation of free and bound hormone for which charcoal, double antibody techniques, etc. are used. Such steps are cumbersome, exacting and time consuming.